The present invention relates to a semiconductor light emitting device such as a light emitting diode (LED) or a laser diode (LD).
Recently, a semiconductor light emitting device such as a light emitting diode (LED) or a laser diode (LD) has been employed as a long-life and small-sized light emitting source, and in particular, the LED has drawn attention as a high intensity light source suitable for a backlight light source of a liquid crystal display apparatus.
Heretofore, in a structure for outwardly extracting the light emitted from an LED, it is considered effective for avoiding total internal reflection at an interface between the LED and a portion corresponding to an external lens as high as that of a light emitting portion or a light introducing portion of the LED.
Therefore, it was conceived to form a lens with a glass or a resin having a high refractive index and to cover a chip with the lens.
However, in order to avoid total internal reflection inside the chip, the chip and the lens are only allowed to have a gap smaller than the wavelength. Accordingly, in view of the mechanical tolerance, it is hardly possible to extracting the light using the lens and chip as configured above.
Use of a transparent adhesive having a high refractive index for filling the gap between the lens with a high refractive index and the chip is conceivable. However, such an adhesive having a refractive index n almost reaching 1.7 has extremely high hardness. It has been already found that emission lifetime of the LED is shortened by use of such a hard adhesive causing distortion due to the difference in thermal expansion coefficient between materials. Furthermore, if the resin is peeled off from the chip and/or the lens, optical output significantly drops owing to fully met total internal reflection condition.
Therefore, at present, an LED with low luminous efficiency, in which part of emitted light is taken into the chip and vanishes, is used. Because if a lens with a high refractive index is aimed to reduce total internal reflection of a chip, light extraction effect is difficult to be improved without an adhesive layer having a higher refractive index than that of the lens.
Generally, epoxy resin or silicone resin is often used to seal a cap and a lens. In a case of a power LED which allows current flow therethrough and which emits heat a lot, in particular, it is considered essential to use semisolid one, that is, grease-like or gel-like, such as silicone to seal around the chip.
Here, a structure of a general high-power LED is shown in FIG. 3.
As shown in FIG. 3, a related art high-power LED 100 has a heat sink 120 disposed at the center of a base 110. A semiconductor light-emitting chip 140 is placed and fixed on a sub-mount 130 on the heat sink 120. A silicone resin 150 is filled in a space between the semiconductor light-emitting chip 140 and a plastic lens 160 covering the semiconductor light-emitting chip 140.
The silicone resin 150 has functions of:
(1) dissipating heat generated from the semiconductor light-emitting chip 140 (having a higher heat conductance than air);
(2) relaxing mechanical stress occurred owing to difference in thermal expansion coefficient between materials; and
(3) breaking total internal reflection condition (Frustrated TIR condition) occurred on a chip surface by optically keeping in contact with the surface of the semiconductor light-emitting chip 140.
See Japanese Patent Application Publication 2004-15063 and Japanese Translation of PCT International Application Hei 7-0507823, for example.